Peroxisome proliferators are a structurally diverse group of compounds which, when administered to mammals, elicit dramatic increases in the size and number of hepatic and renal peroxisomes, as well as concomitant increases in the capacity of peroxisomes to metabolize fatty acids via increased expression of the enzymes required for the β-oxidation cycle (Lazarow and Fujiki, Ann. Rev. Cell Biol. 1:489-530 (1985); Vamecq and Draye, Essays Biochem. 24:1115-225 (1989); and Nelali et al., Cancer Res. 48:5316-5324 (1988)). Compounds that activate or otherwise interact with one or more of the PPARs have been implicated in the regulation of triglyceride and cholesterol levels in animal models. Compounds included in this group are the fibrate class of hypolipidemic drugs, herbicides, and phthalate plasticizers (Reddy and Lalwani, Crit. Rev. Toxicol. 12:1-58 (1983)). Peroxisome proliferation can also be elicited by dietary or physiological factors such as a high-fat diet and cold acclimatization.
Biological processes modulated by PPAR are those modulated by receptors, or receptor combinations, which are responsive to the PPAR receptor ligands. These processes include, for example, plasma lipid transport and fatty acid catabolism, regulation of insulin sensitivity and blood glucose levels, which are involved in hypoglycemia/hyperinsulinemia (resulting from, for example, abnormal pancreatic beta cell function, insulin secreting tumors and/or autoimmune hypoglycemia due to autoantibodies to insulin, the insulin receptor, or autoantibodies that are stimulatory to pancreatic beta cells), macrophage differentiation which lead to the formation of atherosclerotic plaques, inflammatory response, carcinogenesis, hyperplasia, and adipocyte differentiation.
Subtypes of PPAR include PPAR-alpha, PPAR-delta (also known as NUC1, PPAR-beta and FAAR) and two isoforms of PPAR-gamma. These PPARs can regulate expression of target genes by binding to DNA sequence elements, termed PPAR response elements (PPRE). To date, PPRE's have been identified in the enhancers of a number of genes encoding proteins that regulate lipid metabolism suggesting that PPARs play a pivotal role in the adipogenic signaling cascade and lipid homeostasis (H. Keller and W. Wahli, Trends Endoodn. Met. 291-296, 4 (1993)).
Insight into the mechanism whereby peroxisome proliferators exert their pleiotropic effects was provided by the identification of a member of the nuclear hormone receptor superfamily activated by these chemicals (Isseman and Green, Nature 347-645-650 (1990)). The receptor, termed PPAR-alpha (or alternatively, PPARα), was subsequently shown to be activated by a variety of medium and long-chain fatty acids and to stimulate expression of the genes encoding rat acyl-CoA oxidase and hydratase-dehydrogenase (enzymes required for peroxisomal β-oxidation), as well as rabbit cytochrome P450 4A6, a fatty acid ω-hydroxylase (Gottlicher et al., Proc. Natl. Acad. Sci. USA 89:4653-4657 (1992); Tugwood et al., EMBO J 11:433-439 (1992); Bardot et al., Biochem. Biophys. Res. Comm. 192:37-45 (1993); Muerhoff et al., J Biol. Chem. 267:19051-19053 (1992); and Marcus et al., Proc. Natl. Acad. Sci. USA 90(12):5723-5727 (1993).
Activators of the nuclear receptor PPAR-gamma (or alternatively, PPARγ), for example troglitazone, have been clinically shown to enhance insulin-action, to reduce serum glucose and to have small but significant effects on reducing serum triglyceride levels in patients with Type 2 diabetes. See, for example, D. E. Kelly et al., Curr. Opin. Endocrinol. Diabetes, 90-96, 5 (2), (1998); M. D. Johnson et al., Ann. Pharmacother., 337-348, 32 (3), (1997); and M. Leutenegger et al., Curr. Ther. Res., 403-416, 58 (7), (1997).
The third subtype of PPAR, PPAR-delta (or alternatively, PPARδ, PPARβ, or NUC1) initially received much less attention than the other PPARs because of its ubiquitous expression and the unavailability of selective ligands. However, genetic studies and recently developed synthetic PPAR-δ agonists have helped reveal its role as a powerful regulator of fatty acid catabolism and energy homeostasis. Studies in adipose tissue and muscle have begun to uncover the metabolic functions of PPAR-δ. Transgenic expression of an activated form of PPAR-δ in adipose tissue produces lean mice that are resistant to obesity, hyperlipidemia and tissue steatosis induced genetically or by a high-fat diet. The activated receptor induces genes required for fatty acid catabolism and adaptive thermogenesis. Interestingly, the transcription of PPAR-γ target genes for lipid storage and lipogenesis remain unchanged. In parallel, PPAR-δ-deficient mice challenged with a high-fat diet show reduced energy uncoupling and are prone to obesity. Together, these data identify PPAR-δ as a key regulator of fat-burning, a role that opposes the fat-storing function of PPAR-γ. Thus, despite their close evolutionary and structural kinship, PPAR-γ and PPAR-δ regulate distinct genetic networks. In skeletal muscle, PPAR-δ likewise upregulates fatty acid oxidation and energy expenditure, to a far greater extent than does the lesser-expressed PPAR-α. (Evans R M et al 2004 Nature Med 1-7, 10 (4), 2004)
PPARδ is broadly expressed in the body and has been shown to be a valuable molecular target for treatment of dyslipidemia and other diseases. For example, in a recent study in insulin-resistant obese rhesus monkeys, a potent and selective PPARδ compound was shown to decrease VLDL and increase HDL in a dose response manner (Oliver et al., Proc. Natl. Acad. Sci. U.S.A. 98: 5305, 2001). Also, in a recent study in wild-type and HDL-lacking, ABCA1−/− mice, a different potent and selective PPARδ compound was shown to reduce fractional cholesterol absorption in the intestine, and coincidently reduce expression of the cholesterol-absorption protein NPC1L1 (van der Veen et al., J. Lipid Res. 2005 46: 526-534).
Because there are three isoforms of PPAR and all of them have been shown to play important roles in energy homeostasis and other important biological processes in human body and have been shown to be important molecular targets for treatment of metabolic and other diseases (see Wilson, et al. J. Med. Chem. 43: 527-550 (2000)), it is desired in the art to identify compounds which are capable of interacting with multiple PPAR isoforms or compounds which are capable of selectively interacting with only one of the PPAR isoforms. Such compounds would find a wide variety of uses, such as, for example, in the treatment or prevention of obesity, for the treatment or prevention of diabetes, dyslipidemia, metabolic syndrome X and other uses.
Several PPAR-modulating drugs have been approved for use in humans. Fenofibrate and gemfibrozil are PPARα modulators; pioglitazone (Actos, Takeda Pharmaceuticals and Eli Lilly) and rosiglitazone (Avandia, GlaxoSmithKline) are PPARγ modulators. All of these compounds have liabilities as potential carcinogens, however, having been demonstrated to have proliferative effects leading to cancers of various types (colon; bladder with PPARα modulators and liver with PPARγ modulators) in rodent studies. Therefore, a need exists to identify modulators of PPARs that lack these liabilities.
Additionally, recent evidence points to a role for PPAR-δ in the development of cancers, including colon, skin, and lung cancers. Modulators of PPAR could therefore find use in the treatment of cancers of various types.